(i) Field of the Invention
The present invention relates to abradable gas turbine seals and, more particularly, to novel turbine seals having cellular metallic structures suitable for operation in an oxidizing and/or carburizing gas environment at high temperatures.
(ii) Description of the Related Art
Cellular structures made from thin sheet metal or metal foil are attractive for use in space and aerospace applications, in particular in jet engines, because they provide a high stiffness to weight ratio and high mechanical energy absorption capabilities and acoustic damping while being light in weight. As a direct consequence of their low density they are also readily abradable, which is a characteristic favorable for usage in jet engines, and also in stationary gas turbines used for power generation, to reduce the gap between the stationary shroud and rotating blade components, thereby improving efficiency of the gas turbine cycle. While the cellular structure used as an abradable seal must be soft enough to allow the tips or knife edges of a rotating blade to cut into it without causing damage to the blade and the structure carrying the abradable cellular structure, it must be strong enough to withstand static and high frequency vibratory loads, abrasion, cyclic thermal stresses and cyclic oxidation and/or carburization attack, all occurring at high temperatures. Furthermore, the seal must not crack under thermal shock and low cycle fatigue loading. In other words, the cellular structure used as a gas path seal must be resistant to hot gas corrosion attack and must have excellent structural integrity and long term dimensional stability to withstand the mechanical and thermal loads imposed on it for many cycles and over lengthy periods of time.
Conventional abradable cellular gas path seals are manufactured from highly alloyed, austenitic stainless steels and nickel-base alloys and are provided having regular hexagonal cells formed by corrugating ribbons of sheet or foil and welding the corrugated sheets together where abutting walls of adjacent corrugated ribbons meet to form a double wall, i.e. a node.
U.S. Pat. No. 3,867,061 issued Feb. 18, 1975 typifies a conventional prior art honeycomb shroud for rotor blades for turbines in which the honeycomb cell walls are made from nickel-base heat-resistant alloys and the honeycomb strips are brazed or resistance welded to a back-up ring.
U.S. Pat. No. 4,063,742 issued Dec. 20, 1977 discloses another embodiment of prior art abradable fluid seal for use in gas turbines consisting of a conventional honeycomb made by conventional honeycomb equipment in which abutting three-sided semi-hexagonal strips each having a pair of flat slanted sides and a flat crest of equal length standing on edge (xe2x80x9csoexe2x80x9d) are resistance welded together at the crests.
Honeycomb structures typically are fabricated by building up the structure layer by layer to result in a three dimensional body having a height determined by the width to which the ribbon had been slit prior to corrugation. The length of the structure is parallel to the plane of the double walls or nodes and the width is represented by the direction of layer build up. Such a cellular body is brazed to face sheets to form a sandwich skin or brazed to backplates of a rind or ring segments to form a seal, the contacting surface of the cellular body being a xe2x80x9csoexe2x80x9d surface. Such brazing does not only join the cellular structure body to the face sheet or backplate but also contributes significantly to the stiffness of the cellular structure itself. This is due to the fact that the brazing alloy, in a liquid state and due to capillary action, rises up the gap formed by the two neighbouring walls of the node, thereby wetting the abutting surfaces of said node walls and, after resolidification of the braze filler metal, forms a stiffened cellular structure. The braze flow up the nodal walls is referred to as xe2x80x9cwickingxe2x80x9d. Such wicking is essential to provide a brazed cellular structure with good mechanical behaviour at high temperatures to resist combined thermal and mechanical loads.
The most commonly used method to provide a turbine engine seal segment or ring is to sandwich a braze filler metal foil, tape or brazing powder between the abutting surfaces of the xe2x80x9csoexe2x80x9d cellular structure and the backplate surface, and brazing this assembly together. The liquid braze metal must travel up the full depth of the nodes to impart improved structural strength. The exposed surface at which the rotor blade tip or knife edge rubs is also the surface where the most severe combination of mechanical load, wear and temperature occurs and therefore requires excellent structural integrity. If inadequate wicking success is achieved during brazing, there is a twofold drawback: not only is the cellular structure unstiffened, with the consequence of low shape stability, which may cause premature failure of the seal body in service, but also the bulk of the braze filler metal remains at the backplate surface and penetrates both the backplate material and the cellular foil alloy structure by diffusion while in the liquid state to a much larger extent than would be the case if the braze alloy flowed up the nodes. This has the effect of significantly altering the chemical and mechanical characteristics of the backplate and the foil metal alloy, at least locally at their juncture.
Because oxidation resistance and carburization resistance at high temperatures are required, sheet or foil metals having a good hot gas corrosion resistance must be used for the manufacture of turbine seals. The resistance of metals to oxidation and carburization is based on the formation of surface oxide layers which protect the underlying metal from further attack. The nickel base alloys and highly alloyed, austenitic stainless steels used in conventional abradable cellular seals rely on the formation of chromia (Cr2O3) or mixed Cr2O3 /NiO oxides to provide such protection. At very high temperatures or in combustion gas atmospheres flowing at high speeds, both found in turbine engines, this type of protection is unstable due to further oxidation of the Cr2O3 to volatile CrO3 as described by James L. Smialek and Gerald E. Meier in Superalloys II by Chester T. Sims et al. (eds.), John Wiley and Sons, Inc. (1987). The same authors, in the same handbook, describe that much better protection is achieved with alumina (Al2O3) which is formed on metals having a high Al concentration and being further enhanced by high chromium (Cr) contents and the addition of rare earth metals, such as yttrium (Y), zirkonium (Zr), cerium (Ce), hafnium (Hf), ytterbium (Yb), praseodymium (Pr), neodymium (Nd), samarium (Sm) or lanthanum (La) leading to so called MCrAlX alloys with X representing the rare earth metal addition and M being the major alloy constituent selected from the group of Ni, Fe or Co or combinations thereof. If yttrium is chosen as the main rare earth addition, then the resulting alloys are referred to as MCrAlY alloys. MCrAlY alloys are disclosed in U.S. Pat. No. 5,116,690 by W. J. Brindley et al., issued May 26, 1992. Other patents such as U.S. Pat. No. 4,034,142 issued Jul. 5, 1977 to R. J. Hecht and U.S. Pat. No. 4,503,122 issued Mar. 5, 1985 to A. R. Nicholls describe similar MCrAlY alloys with excellent self protection against hot gas attack. All the aforementioned patents describe the use of MCrAlY alloys as overlay coatings and not as a structural material in He form of foil to provide a welded cellular structure.
It is difficult to obtain MCrAlY alloys in thin sheet or foil form because they are hard and difficult to roll which is the effect of the high aluminium concentration, typically in the range 2-6% by weight, with 6xe2x80x37% representing the upper limit to retain workability. If available in thin sheet or foil form the MCrAlY materials are difficult to corrugate and to form into a cellular structure such as described above. In particular, these materials are difficult to form into a corrugated ribbon, if a portion or all of the added yttrium is present as yttria (Y2O3) and/or part of the alloy matrix aluminium is present as matrix alumina. The insufficient formability of these alloys results in cellular structures which may deviate significantly from the optimum shape since no sharp comers, but only rounded ones with a relatively large bend radius, can be achieved by corrugations which compromises brazeablity. This is especially true for MCrAlY foil or sheet metal having thick gauges. For use at high temperatures, the foil thickness of the MCrAlY sheet or foil used must be greater than a certain minimum limit to avoid break-away oxidation. Break-away or catastrophic oxidation occurs when, due to straightforward growth of the protective alumina scale or due to repeated scale spallation and automatic rebuild of the protective scale in oxidising environments at high temperature, the bulk aluminium concentration in the foil or sheet alloy is consumed and falls below a certain critical value. This phenomenon is described by W Quadakkers and K Bongartz in Werkstoffe und Korrosion 45, 232-241 (1994). The same authors propose to use high initial Al concentrations in the MCrAlY alloy and the use of thicker foil or sheet to delay the onset of break-away oxidation. Both of these measures, however, are detrimental to formability and to brazeability of the material when formed into a cellular structure and brazed to a backing sheet metal ring, sheet metal ring segments or cast backing members.
Even if successfully formed into a cellular shape with good geometrical features, the MCrAlY materials are difficult to braze because they contain a high amount of Al and Y or Y2O3. Due to the high affinity of aluminium to oxygen, there is a strong tendency towards the formation of stable and tightly adherent alumina scales at the MCrAlY metal surface, thereby reducing the wettability and consequently braze wicking which is required to achieve structural stiffness of the cellular structure to be used as an abradable turbine engine seal. Likewise yttrium has a very strong affinity to oxygen to form very stable yttria (Y2O3) which also acts as a braze flow stopper. Typically therefore MCrAlY alloys, typically containing 6-30% by weight Cr, 2-7% by weight Al, 0.005-0.6% by weight Y and other reactive elements from the group consisting of Zr, Ti, Hf, La, Ce, Er, Yb, Pr, Nd, Sm, balance one or more of the elements belonging to the group of Fe, Ni, Co are extremely difficult to braze and it is therefore difficult to use them in a cellular structure of an abradable seal system.
H. Bode in xe2x80x9cMetal-Supported Automotive Catalytic Convertersxe2x80x9d, H. Bode (ed. ), Werkstoff Informationsgesellschaft mbH, Frankfurt (1997), p. 17-31, describes cellular structures made from MCrAlY foil alloys for use at high temperature as support structures for automotive catalysts. The cellular structures are built up by alternating layers of flat or microcorrugated foil and corrugated foil having sinusoidal ridges. The sinusoidal corrugated foil may be manufactured using Fexe2x80x94Crxe2x80x94Alxe2x80x94Y alloys.
PCT Application No. PCT/EP95/00885 (WO 95/26463)published Mar. 9, 1995, discloses a metallic cellular structure made from an MCrAlY alloy having an aluminium content of greater than 6% by weight for increased electrical resistivity. The cellular structure is fabricated by extrusion of metal powders or metalxe2x80x94ceramic powders or by making the metal foil by rapid solidification because of the difficulties in formability of MCrAlY alloys having an aluminum content of greater than 6wt %.
In accordance with the present invention there are provided novel cellular structures preferably made from MCrAlY alloy metal foil or sheet having good structural integrity and stiffness after brazing to a metal backing structure and therefore show long term dimensional stability at high temperature.
In its broadest form this is achieved through a novel, elongated cell shape of a cellular honeycomb structure. According to the present invention, the cell shape is elongated in the direction parallel to the direction of the double wall crests or nodes and, more particularly, comprises a plurality of abutting semi-hexagonal strips each having alternating flat slanted sides interconnected by a flat crest, the abutting strips joined together at the adjacent flat crests to form generally hexagonal cells having double wall crests or nodes, the slanted sides having an equal length and the crests having a length greater than slanted sides whereby the width w measured between opposed flat crests of adjacent strips relative to the distance b between the planes of opposed slanted sides has a ratio of b:w of greater than 1.15:1.0. Preferably, the ratio of b:w is 1.2 to 2.0:1.0, and snore preferably, the ratio of b:w is 1.3 to 1.6:1.0.
In a further preferred embodiment of the invention, the semi-hexagonal elongate strips are a foil metal or sheet metal alloy of a MCrAlY comprising 13 to 27% by weight Cr, 2 to 7% by weight Al, 0.005 to 0.6% by weight Y, at least one of up to 0.6% by weight Zr, up to 0.6% by weight Hf, up to 0.6% by weight Ce, up to 6% by weight La, up to 6% by weight Si, up to 0.6% by weight Mn, up to 0.6% by weight Ti and up to 0.3% by weight C, and the balance, apart from impurities, Fe or Ni or combinations thereof. More preferably, the iron content is at least 6% by weight Fe, the balance Fe or Ni or combinations thereof.
The novel honeycomb structure of the invention can be used as an abradable seal in a gas turbine such as a jet engine or stationary gas turbine comprsing a metal backplate with the honeycomb structure attached standing on edge to the metal backing plate, the honeycomb structure consisting of the metal foil or sheet metal of the MCrAlY. The metal back member preferably is a nickel-base alloy in the form of a backing sheet metal ring, sheet metal ring segments or cast backing members and the metal foil or sheet metal used to manufacture the cellular structure has a thickness of 0.100 mm to 0.400 mm.